In most cases early detection of slow leaks and development of the deflated condition in the tire of a moving vehicle to a large extent depends on the personal ability of the driver to feel minor changes in the vehicle riding qualities resulting from a loss of pressure in the tire.
The driver is usually aware of deflation only after the pressure in the tire is already lost to such an extent that changes in the vehicle riding qualities are accompanied by difficulties in vehicle control, which might even cause a safety hazard.
There are also other undesirable consequences of the deflated condition of a tire, e.g., excessive flexing and heating of the tire followed by its rapid wear and often irreparable damage.
This situation becomes especially critical in the case of heavy trucks with multiple tires, or truck tractors with articulated vehicles liked towed trailers or platforms, in that there are many specific factors relating to such vehicles which make working conditions particularly unfavorable for early detection of minor changes in vehicle behaviour, i.e., at the very beginning of deflation.
Among the factors which prevent the driver's timely detection of deflation development should be mentioned: increased level of noise and vibrations during riding, distance disposition of the truck tractor and the towed vehicle, excessive loads on the front axle caused by the front drive assembly absorbing changes in vehicle behaviour before they are felt by the driver, etc.
There are numerous known systems which have been designed to solve the above problem and which are intended to warn the driver about the low pressure condition in pneumatic tires of a travelling vehicle.
These systems can generally be divided into two types:
1) those including a sensing means directly mounted on the rotating tire and able to detect changes in the tire caused by deflation, e.g., generation of excessive heat or deficient pressure in the tire. An appropriate signal, generated by a sensing means informs the driver of these changes. PA1 2) those including a feeler member which touches the ground when the distance between the vehicle and the road surface is reduced due to the deflated condition of the tire, and then activates an appropriate signal-producing means which warns the driver. PA1 a system for detection, monitoring and warning of reduction in the air pressure in a wheel tire of a traveling vehicle, provided with at least one pneumatically inflated tire, when said tire engages the road surface, consisting of: PA1 characterized in that PA1 at least part of said post is made resiliently bendable as to be able to bow in any direction when it contacts the road surface while being dragged by the moving vehicle, when said distance is sufficiently reduced and to return to its initial configuration after said distance is, at least partially, restored; said sensing means is adapted to detect a condition when the post is dragged; the system comprises a timing means, counting time interval between each consequent off- and on-condition of said switching means, which corresponds respectively to dragged/non-dragged condition of the post and said timing means is adapted to activate said signal producing means, when said time interval exceeds preset value.
A description of the systems relating to the first type can be found, for example, in Saint's U.S. Pat. No. 4,970,491 and in Wang's U.S. Pat. No. 5,001,457.
The system disclosed in Saint includes a transmitter, coupled with the valve stem of each vehicle tire, which senses a respective low pressure condition of the tire, and which transmits an appropriate signal, accordingly activating an alerting means.
The system described in Wang consists of a plurality of pressure gauges, each respectively attached to pneumatic tires and each pressure gauge transmitting a signal to the appropriate alarm circuit when a certain tire is deflated below a predetermined nominal pressure.
There are some inherent disadvantages related to this type of warning systems, in general, and with those disclosed in the above patents, in particular. Among these disadvantages one can mention insufficient reliability associated with the possibility of the sensing means affixed to the rotating tire to be damaged due to centrifugal forces acting upon it. Another disadvantage which can be referred to this type of warning system is the inconvenience in maintenance due to the necessity of dismantling the sensing means from the deflated tire and remounting it on a new one each time replacement of a deflated tire takes place.
The present invention relates to the second type of warning system referred to above, which is based on utilization of a feeler member and its contact with the ground to generate an alarm signal. Descriptions of such a system can be found in Clothier's U.S. Pat. No. 4,075,602 and Sweet's U.S. Pat. No. 5,032,822.
The low tire alarm system disclosed in Clothier's patent includes a feeling member extended in a downward position and mounted on a shaft in the vicinity of each tire, and pivot movement of said member is possible with respect to the shaft.
The feeler member is normally situated above ground level when the associated vehicle tire is in a normally inflated condition. However, when there is a low pressure condition in one or more of the ground-engaging tires, contact is made between the feeler member and the ground surface. The feeler member is forced by the forward moving vehicle to pivot about the shaft, and this causes the electrical circuit to light the lamp which warns the driver that a low pressure condition exists in one of the vehicle tires.
The tire deflation warning system specified in Sweet's patent utilizes a device which is attached to the wheel axle of a vehicle, adjacent to each pneumatically inflated tire at a fixed distance from the underlying road surface. This device employs a vertically disposed post-like feeler member, its lower extremity adapted to contact the road surface when the distance between the road surface and wheel axle is reduced due to tire deflation. When the post touches the road surface, it is forced upward, thus causing contact to be made with an electrical switch, which in turn activates a signal-producing means in the driver's cabin.
One of the disadvantages associated with the systems referred to above is that the feeler member employed in them is made rigid, in order to ensure proper activation of the signal-producing means when this feeler member touches the road surface.
Since this rigid feeler member is mounted under the wheel axle and extends relatively far downward toward the ground, there is a danger that it might easily be damaged by obstacles which protrude above the ground, and which are high enough to collide with its lower extremity.
Collision might happen even before deflation starts and the feeler member touches the ground. A similar situation might happen when the vehicle returns from the low road side to the main road.
Another disadvantage of known warning systems lies in the fact that in these systems the distance between the lower extremity of the feeler member and the road surface depends not only on the tire condition, but is also influenced by the loading condition of the undercarriage. This factor is especially pronounced in heavy trucks, truck tractors, towed trailers and platforms, in which said distance might be reduced by 3,5-7 centimeters in loaded condition, as compared to not loaded condition.
The above consideration requires reservation of additional space between the feeler member and the road surface in order to ensure that they will not touch when the vehicle is loaded, and there is still no tire deflation. An inevitable consequence of this requirement is the possibility that deflation in the tire will be detected only after significant loss of pressure.
Still another disadvantage associated with known lower pressure warning systems relates to the fact that they only alert with regard to deflation but are not able to predict tire behaviour after deflation is detected.
In conclusion, it should be emphasized that despite the fact that the above problem of deflation detection in vehicle tires has been known since the very beginning of the introduction of pneumatically inflated tires, it is still a vital one and there is still a need for a new, reliable, convenient and economical system, which will ensure the early detection of deflation, its monitoring and timely warning to the driver when this condition might become hazardous.